Shielded electrical cable



April 1968 0. cs. GARNER 3,379,821

SHIELDED ELECTRICAL CABLE Filed Jan. 22, 1965 AREA OF CONTROLLEDPOLYE'THYLENE F 2 LEssER ADHESION (MATERIAL 0 0A L E 113531-233? 2 L1METALLIC V 25 STRIP 24 POLYETHYLENE HAVING ADHESIVE 4 52 LESSER ADHESIVEABILITY AREA METALLIC STRIP EXTRUDED A CABLE CORE APPARATUS SHIELDINGFOR FOLDING STRIP Q SHIELDING STRIP EXTRUDER ABOUT CABLE CORE OXIDIZINGFLAME OF SHIELDING STRIP CABLE com:

\ APPARATUS RELEASE FOR FOLDING SHIELDING STRIP I I XTR ABOUT CABLEAPPL'C L 7 CORE L.

I INVENTOR T P?P F 6 OSCAR e. GARNER 5% QMM,MQM

ATTORNEYS.

United States Patent 3,379,821 SHIELDED ELECTRICAL CABLE Oscar G.Garner, Westlicld, Ni, assiguor to General Cable Corporation, New York,N.Y., a corporation of New Jersey Filed Ian. 22, 1965, Ser. No. 427,43512 Claims. (Cl. 174-46) This invention relates to improvements inshielded and jacketed electrical cables. More particularly the inventionrelates to such cables in which the shield comprises a metallic strip,coextensive in length with the cable core, which is folded about thecore into the form of a tube, and in which there is an outer jacket ofpolyethylene extruded over the tubular shield so formed. The metallicstrip conventionally is corrugated transversely prior to being foldedabout the cable core in order to give greater flexibility to the cableand to permit bending of the completed cable without wrinkling orrupture of the shielding layer, In the cable to be described herein thelongitudinal edges of the folded metallic strip overlap each otherslightly to insure complete coverage of the enclosed cable core. Thepolyethylene jacket is extruded over the tubular shield at an elevatedtemperature, for example in the range of 400-450 F. for high molecularweight polyethylene and for extrusion speeds of 50 feet per minute, andup. As the extruded polyethylene jacket cools the polyethylene contractsand tends to squeeze the tubular shield down firmly onto the cable core.Telephone cable of this construction is known as alpeth cable and isdescribed more fully in the F. W. Horn and R. B. Ramsey paper, BellSystem Cable Sheath Problems and Designs, in AIEE Proceedings, 1951,volume 70.

In conventional plastic telephone cables, that is in cables in which theinsulation on the conductors is a plastic material such as polyethylene,aluminum strip has been used for many years as the shielding material,for example a transversely corrugated aluminum strip of 8 mils inthickness, over which is extruded a polyethylene packet of the order of80 mils in thickness, When the polyethylene is extruded onto the tubularshield formed by the folded aluminum strip there is no adhesion of thepolyethylene to the aluminum. As the polyethylene cools from 400 F. to80 F. it contracts 21% in volume. The cooling proceeds from the outersurface of the jacket inwardly and the resulting shrinkage of the jacketforces the polyethylene into close engagement with the corrugatedsurface of the folded strip and then causes the tubular shield to bepartially collapsed by the sliding of the overlapping edges of the striprelative to each other. This collapse of the tubular shield is haltedwhen the contracting tube becomes supported by the enclosed cable core.In this shrinking process the outer overlapping edge of the folded strippresses outwardly into the plastic jacket, making the jacket thinneralong the strip edge. Such thinning is undesirable. It has been proposedin the L. Iachimowicz Patent 3,087,007, Apr. 23, 1963, to reduce suchthinning by the use of a longitudinal bridging tape over the overlappededges of the strip.

While the bridging tape is effective in minimizing or preventinglocalized thinning of the polyethylene jacket at the overlapping edgesof the folded metallic strip, it does not provide a hermetic seal at theoverlapped edges. In the event of damage to the jacket by lightning orby mechanical action moisture may penetrate into the cable core betweenthe overlapped edges.

3,370,$2l Patented Apr. 23, 1968 To improve the corrosion resistance ofthe shield a special adhesive polyethylene film may be applied to coverone or both sides of the metallic strip. Such an adhesive polyethylenefilm is disclosed in the L. Jachimowicz Patent No. 3,233,036, assignedto the owner of the present application. The polyethylene used for thisfilm contains reactive carboxyl groups which have the ability to developfirm adhesion to the metallic strip and also to the overlyingpolyethylene jacket. Aluminum strip protected by the adhesivepolyethylene film is highly resistant to corrosion. In addition, use ofthe adhesive polyethylene film improves telephone cables by inhibitingpenetration of moisture into the cable core. This improved resistance tomoisture penetration results in part from the sealing of the seam at theoverlapping edges of the polyethylene coated shielding strip, therebyforming a tight tube or pipe around the core of the cable. Theresistance of this cable structure against the penetration of moistureis further improved by reason of the sealing of the outer polyethylenejacket to the adhesive polyethylene coated shielding tape resulting fromthe use of sufficiently high extruding temperature and pressure toachieve optimum bonding. An added advantage resulting from use of theshielding strip with the sealed seam is the better mechanical strengthof the cable structure and improved life under repeated bending.

In the cable structure just described there is an intimate bond betweenthe outer polyethylene jacket and the shielding tape over their entireinterface. The firm adhesion between the extruded polyethylene jacketand the shielding layer poses a problem for cable termination whichrequires the grounding of the metallic shielding strip. The same is truewhen two lengths of cable are to be spliced together. It is cumbersomeand difficult to separate the jacket from the metallic shielding layerfor the purpose of making electrical connection to the shield.Manufacture of the cable with something less than an intimate bondbetween the jacket and the shield will make easier grounding atterminations and easier splicing of cables while still providingexcellent corrosion protection for the aluminum shielding tape. However,reduction in the bonding over the interface will be at the sacrifice ofabsolute resistance to moisture penetration which can be obtained onlyif the seam of the folded shielding strip is sealed and if the overlyingpolyethylene jacket is firmly bonded to the shielding layer,particularly at the sealed seam of the shielding layer.

The somewhat contradictory requirements of absolute resistance tomoisture penetration and, at the same time, comparative ease ofgrounding for the shielding layer are achieved by the improvedconstruction and method of the present invention. My new constructionutilizes the aluminum or other metal shielding strip precoated withspecial adhesive polyethylene, preferably on both sides, with the outerpolyethylene jacket extruded under such temperature and pressureconditions as to obtain firm adhesion over a portion only of theinterface between the extruded jacket and the coated shielding layer.According to the present invention firm adhesion is restricted to thearea of the interface over and adjoining the sealed seam of the foldedmetallic strip, while leaving the balance of the area between theextruded jacket and the precoated folded shielding strip easilystrippable. This spatially limited adhesion between the extruded jacketand the shielding layer may be accomplished by a number of the methodswhich will be described hereinafter.

It is an object of this invention to provide an improved shielded andjacketed cable construction and method of manufacture. It is a furtherobject of the invention to provide shielded and jacketed cable in whichthe shield, formed from a metallic strip folded about the cable corewith overlapping edges, is hermetically sealed and is firmly bonded tothe overlying jacket at the overlap and for a limited peripheraldistance on both sides thereof, While having a controlled lesser bond tothe jacket around the rest of the periphery.

The drawing forming a part hereof show preferred embodiments of theinvention selected for illustrative purposes only. The legends on thedrawings are not limitations on the invention, which is defined by theclaims, but are merely for convenience in the consideration of theillustrative embodiments of the invention. In the drawings:

FIGURE 1 is a perspective view of a short length of shielded andjacketed cable made according to the present invention, the jacket beingcut back to show the construction better;

FIGURE 2 is a transverse cross section, to enlarged scale as compared toFIGURE 1, through a coated metallic strip suitable for use to form theshielding layer in the cable;

FIGURE 3 is a transverse cross section through the cable of FIGURE 1,substantially on line 3-3, and to somewhat enlarged scale;

FIGURE 4 is a diagrammatic view showing how metallic shielding stripcoated with highly adhesive polyethylene may be treated to selectivelycontrol the adhesion during manufacture of the cable;

FIGURE 5 is a fragmentary transverse section through a cable sheathillustrating another way of selectively controlling the adhesion of thepolyethylene jacket to the shielding layer; and

FIGURE 6 is a diagrammatic view showing one way of making theconstruction of FIGURE 5.

Referring first to FIGURE 1, the cable core 11 comprises a plurality ofinsulated conductors, for example plastic insulated wires arranged inpairs. The precise nature of the cable core is not material to thepresent invention and further description is unnecessary to anunderstanding of this invention. Enclosing the cable core is the shield12 comprising a transversely corrugated metallic strip coextensive inlength with the cable core and folded about the core into a tube withthe longitudinal edges of the strip overlapping each other a shortdistance, as shown. Over the tubular shielding layer is an extrudedjacket 13 of polyethylene. Prior to being folded about the cable corethe metallic strip 12 was precoated, preferably on both sides, with afilm of an adhesive polyethylene containing reactive carboxyl groups.This coating material has the ability to develop a firm adhesive bond tothe metallic strip and also to the polyethylene jacket 13 extruded overthe shielding layer. Details of the coated metallic strip 12 aredisclosed in other figures of the drawings. Extrusion of thepolyethylene, hot and under pressure, over the folded shielding stripproduces a hermetic seal between the overlapping edges of the coatedstrip and a firm adhesion of the polyethylene jacket to the shield inthe area over and adjoining the sealed seam. This area, extendingbetween the longitudinal marks A and B shown in FIGURE 1, preferably islimited to less than one-half the periphery or circumference at theinterface. In the illustrative embodiment the peripheral extent of thefirm adhesion is shown as about one-fourth the periphery.

By treatment in the manner hereinafter described a controlled lesseradhesion is obtained between the extruded jacket and the metallic shieldat the interface around the rest of the periphery, this area beingindicated by the arrow 15 in FIGURE 1. The result is a cableconstruction in which the seam at the overlapping edges of the shieldingstrip is hermetically sealed and in which the enclosed polyethylenejacket is firmly adhered to the shield over the seam and for a limitedperipheral dis- CJI tance on both sides thereof. Around the rest of theperiphery the polyethylene jacket is adhered to the shielding layer lessfirmly, thus facilitating removal of the jacket in this area to exposethe metallic strip for making an electrical connection thereto.

FIGURE 2 is a transverse section to enlarged scale through a coatedmetallic strip suitable for use in forming the shielding layer. InFIGURE 2 the metallic strip 21 is coated on both surfaces with films ofpolyethylene material. For purposes of illustration the thickness of themetallic strip and the coating layers are exaggerated. In practice, analuminum shielding strip might be from 5 to 10 mils thick and the filmcoating layer might be from 1 to 3 mils thick. This coating material 22along both edges of the strip, in the limited areas designated 23 and24, is polyethylene containing a reactive carboxyl group which has theability to develop firm adhesion to the metallic strip, to itself at theoverlapped edges of the strip, and also to the polyethylene jacket whenit is extruded thereover. The coating material 25 along the centerportion 26 of the strip has a controlled lesser ability to develop firmadhesion to the metallic strip and to the polyethylene jacket which willbe extruded over the shielding layer.

FIGURE 3 is a cross section taken transversely through the cable ofFIGURE 1, the thickness of the metallic shielding strip and the coatinglayers thereon being exaggerated for purposes of illustration. Asexplained hereinabove, the film coating along the edges of the metallicstrip has the ability to develop firm adhesion between the overlappingedges of the folded strip, and to the polyethylene jacket in the regionover the seam and for a limited peripheral distance on both sidesthereof. This results in a hermetic seal along the longitudinal seam ofthe folded shielding strip. The film coating along the center portion ofthe metallic strip has a controlled lesser ability to develop adhesionto the polyethylene jacket around the peripheral extent 15 of thetubular shield. Thus, while there preferably is adherence of the filmcoating to the metallic strip and to the overlying jacket in this regionit is less than along the seam of the shielding layer. As a result,removal of the polyethylene from the center portion of the metallicstrip to permit electrical connection thereto is facilitated with littledanger of stripping the polyethylene from the metallic strip along theseam at its overlapping edges.

The precoated metallic strip illustrated in FIGURE 2 may be formed byextruding the polyethylene films onto the strip. During such extrusionpolyethylene of limited adhesive characteristic may be extruded over thecenter part of the strip and highly adhesive polyethylene over the twoedge portions of the strip.

Alternatively, highly adhesive polyethylene may be extruded over theentire Width of the metallic strip and the center portion of the stripthen treated to selectively reduce its adhesive ability. I have foundthat the adhesiveness of a coating of polyethylene containing reactivecarboxyl groups on the metallic shielding strip is greatly reduced by anoxidizing flame treatment. Apparently the ability of the carboxyl groupsrequired to develop high adhesive properties is made less effective bysuch treatment. By treating the center part of an aluminum strip coatedwith polyethylene containing reactive carboxyl groups coated with anoxidizing flame and then using this strip as a cable shielding tape itis possible to obtain excellent sealing in the seam to itself, and tothe polyethylene jacket extruded over the shielding layer, and yetprevent such firm adhesion between the jacket and that part of theshielding tape which has been flame treated. For example, with a threeinch wide polyethylene protected aluminum strip it is possible to flametreat the center part of the strip and leave the edges on either side ofthe strip with the original ability of the film to adhere to thealuminum and to itself, as well as to the outer polyethylene jacket.Flame treatment of the center part of the film coated metallic strip maytake place as the strip enters the apparatus for folding the strip aboutthe cable core, as illustrated in FIGURE 4. Alternatively, the stripcould be pretreated to limit the adhesive ability of the center portionof the strip, if desired.

It also is possible by the use of a suitable release agent, for examplea pigment such as talc, or an oil such as a silicone oil, to limit thearea of firm adhesion between the sealed shield and the polyethylenejacket extruded over it. If the cable core with its enclosinglongitudinally folded metallic strip coated with polyethylene containingreactive carboxyl groups is fed into the extruder with the seam of theshield in the top position, as shown in FIG- URE 6, talc or otherrelease agent may be dusted or wiped around the lower half orthree-quarter of the cable core circumference just before the coreenters the extruder for application of the outer polyethylene jacket.This will result, under proper temperature and pressure conditions, in asealing of the seam and a firm bond on the top half or top quarter ofthe core circumference at the interface between the shield and thejacket, with comparatively easy stripping of the bottom half orthree-quarters of the jacket. This cable construction is illustrated inthe fragmentary cross section of FIGURE 5, where 31 is the overlappingfilm coated metallic strip which encloses the cable core, 13 is theextruded polyethylene jacket, and 32 represents the thin coating ofrelease agent. The release agent, shown of exaggerated thickness merelyfor purposes of illustration, does not extend over the top approximatelyone-quarter of the tubular shielding layer, thus insuring firm adhesionof the polyethylene jacket 13 to the shielding layer at the longitudinalseam and for a limited peripheral distance on both sides thereof.Silicone oil, silicone oil emulsion in water, or a tale dispersion in anoil may be applied in similar manner as release agent to achieve thesame results. Of course, the release agent could be applied by dusting,brushing or wiping to the center portion of that surface of the filmcoated metallic strip which will form the exterior of the tubularshielding layer prior to folding the strip about the cable core.

Preferred embodiments of the invention have been illustrated anddescribed, but changes and modifications can be made without departingfrom the invention as defined in the claims.

I claim:

1. A shielded electrical cable comprising at least one insulatedconductor of indefinite length, a metallic strip which is coextensive inlength with the conductor and which is folded about the insulatedconductor with its edges overlapping each other to provide a tubularmetallic shielding layer completely enclosing the insulated conductor,an outer jacket of plastic insulating material surrounding and closelyenclosing the tubular shielding layer, the plastic jacket being firmlyadhered to the shielding layer along its overlapping edges and for alimited peripheral distance on both sides thereof to seal the overlap,the plastic jacket having a controlled lesser adhesion to the shieldinglayer around the rest of its periphery to facilitate stripping theplastic from the shielding layer in a region removed from the sealedoverlapped region.

2. An electrical cable according to claim 1 in which the outer jacket ispolyethylene and the metallic shielding strip is an aluminum striphaving bonded to its outer surface a coating of a copolymer of ethylenewith monomers containing carboxyl groups, and in which a release agentis interposed between the tubular metallic shielding layer and thepolyethylene jacket for a limited peripheral extent opposite theoverlapped edges of the shielding strip.

3. An electrical cable according to claim 2 in which the release agentcomprises a silicone oil.

4. An electrical cable according to claim 3 in which the release agentcomprises a silicone oil emulsion in Water.

5. An electrical cable according to claim 2 in which the release agentcomprises talc.

6. An electrical cable according to claim 5 in which the release agentcomprises a talc dispersion in oil.

7. An electrical cable according to claim 1 in which the outer jacket ispolyethylene and the metallic shielding strip is an aluminum striphaving bonded to both surfaces thereof coatings of a copolymer ofethylene with monomers containing carboxyl groups.

8. An electrical cable according to claim 1 in which the outer jacket ispolyethylene and the metallic shielding strip is an aluminum striphaving bonded to its outer surface a coating of a copolymer of ethylenewith monomers containing carboxyl groups, the aluminum strip beingcoated along its overlapped edge portions with such material having highadhesive characteristics and along its center portion opposite theoverlap with such material having relatively lesser adhesivecharacteristics.

9. In the manufacture of electrical cable having at least one insulatedconductor, an enclosing tubular metallic shielding layer formed from analuminum strip having bonded to its outer surface a coating of acopolymer of ethylene with monomers containing carboxyl groups, and anouter jacket of polyethylene, the method which comprises treating thecenter portion only of the coated aluminum strip to reduce the adhesioncharacteristics of the coating material along the center of the stripwithout affecting the high adhesion characteristics of the coatingmaterial along the edge portions of the strip, folding the coatedaluminum strip longitudinally around the insulated conductor with itsedges overlapping each other to enclose the insulated conductorcompletely, and extruding the outer jacket of polyethylene over thefolded strip with resulting high adhesion of the jacket to the shieldinglayer along the overlapped portion and lesser adhesion of the jacket tothe shielding layer opposite the overlapped portion.

10. In the manufacture of electrical cable having at least one insulatedconductor, an enclosing tubular metallic shielding layer formed from analuminum strip having bonded to its outer surface a coating of acopolymer of ethylene with monomers containing carboxyl groups, and anouter jacket of polyethylene, the method of controlling the adhesion ofthe jacket to the shielding layer which comprises heating the centerportion only of the coated aluminum strip to reduce the adhesioncharacteristics of the coating material along the center of the stripwithout affecting the high adhesion characteristics of the coatingmaterial along the edge portions of the strip, folding the coatedaluminum strip longitudinally around the insulated conductor with itsedges overlapping each other to enclose the insulated conductorcompletely, and extruding the outer jacket of polyethylene over thefolded strip with resulting high adhesion of the jacket to the shieldinglayer along the overlapped portion and lesser adhesion of the jacket tothe shielding layer opposite the overlapped portion.

11. The method of claim 10 which comprises heating the center portion ofthe coated aluminum strip by subjectin g it to an oxidizing flame.

12. In the manufacture of electrical cable having at least one insulatedconductor, an enclosing tubular metallic shielding layer formed from analuminum strip having bonded to its outer surface a coating of acopolyrner of ethylene with monomers containing carboxyl groups, and anouter jacket of polyethylene, the method of controlling the adhesion ofthe jacket to the shielding layer which comprises folding the coatedaluminum strip longitudinally around the insulated conductor with itsedges overlapping each other to enclose the insulated conductorcompletely, applying a release agent to a peripheral portion only of theshielding layer opposite the overlapped edges to reduce the adhesioncharacteristics of such portion of the coating material withoutaffecting the high adhesion characteristics of the coating materialalong the overlapped edges, and extruding the outer jacket ofpolyethylene over the folded strip with resulting high adhesion of thejacket to the shielding layer along the overlapped portion and lesseradhesion of the jacket to the shielding layer opposite the overlappedportion.

References Cited UNITED STATES PATENTS 3,233,036 2/1966 Jachimowicz17436X 8 FOREIGN PATENTS 942,730 11/1963 Great Britain. 968,061 8/1964Great Britain.

5 DARRELL L. CLAY, Primary Examiner.

A. T. GRIMLEY, Assistant Examiner.

1. A SHIELDED ELECTRICAL CABLE COMPRISING AT LEAST ONE INSULATEDCONDUCTOR OF INDEFINITE LENGTH, A METALLIC STRIP WHICH IS COEXTENSIVE INLENGTH WITH THE CONDUCTOR AND WHICH IS FOLDED ABOUT THE INSULATEDCONDUCTOR WITH ITS EDGES OVERLAPPING EACH OTHER TO PROVIDE A TUBULARMETALLIC SHIELDING LAYER COMPLETELY ENCLOSING THE INSULATED CONDUCTOR,AN OUTER JACKET OF PLASTIC INSULATING MATERIAL SURROUNDING AND CLOSELYENCLOSING THE TUBULAR SHIELDING LAYER, THE PLASTIC JACKET BEING FIRMLYADHERED TO THE SHIELDING LAY-